1. Technical Field
Various embodiments of the present disclosure relate to nonvolatile memory devices and methods of fabricating the same and, more particularly, to unit cells and cell arrays of nonvolatile memory devices and methods of fabricating the same.
2. Related Art
Semiconductor memory devices are typically categorized as volatile memory devices or nonvolatile memory devices. Volatile memory devices lose their stored data when their power supplies are interrupted but have relatively high operating speeds, for example, they read out data stored in memory cells or write data into the memory cells relatively quickly. In contrast, nonvolatile memory devices retain their stored data when their power supplies are interrupted but tend to operate at lower speeds. Therefore, nonvolatile memory devices are used in electronic systems that need to retain data without having a constant power source. Nonvolatile memory devices include mask read only memory (MROM) devices, programmable read only memory (PROM) devices, erasable programmable read only memory (EPROM) devices, electrically erasable programmable read only memory (EEPROM) devices, flash memory devices, etc.
In general, the MROM devices, the PROM devices, and the EPROM devices need additional equipment such as, a UV irradiator, to erase their stored data. Thus, it may be inconvenient to use MROM devices, PROM devices, and EPROM devices in many applications. In contrast, EEPROM devices and flash memory devices allow data to be electrically erased and written without additional equipment. Accordingly, EEPROM devices and flash memory devices may be applied in various areas, for example, systems for program executions or auxiliary memory devices necessitating frequent data renewals. In particular, flash memory devices may be simultaneously erased in units such as, in pages, and are capable of achieving higher integration densities than EEPROM devices. Therefore, flash memory devices are often used in large capacity auxiliary memory devices.
The amount of data that nonvolatile memory devices are capable of storing in each memory cell depends on the number of bits that are stored in each memory cell. A memory cell in which a single bit of data is stored is referred to as a single bit cell or a single level cell (SLC). In contrast, a memory cell in which multi-bit data, for example, data including two bits or more, is stored, is referred to as a multi-bit cell, a multi-level cell (MLC) or a multi-state cell. As semiconductor memory devices become more highly integrated, nonvolatile memory devices employing MLCs have garnered the attention of the semiconductor industry.
Flash memory and EEPROM devices generally have a stacked gate structure including a floating gate and a control gate electrode, which are vertically stacked. However, if the distance between the memory cells is reduced too much, threshold voltages of the memory cells may become unstable due to interference effects or coupling capacitances between the memory cells. Therefore, a lot of research and development goes into perfecting how memory devices can more effectively store data using charge trapping layers.